Fair Value Disclosures - Quantitative Information (Details) - Fair Value, Measurements, Recurring [Member] - Fair Value, Inputs, Level 3 [Member] $ in Thousands	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 6,470,023	$ 6,003,211	$ 5,874,075	$ 5,508,576
USDA Securities [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	9,487	9,999	11,558	13,515
Available-for-sale Securities [Member] | Farmer Mac Guaranteed Securities [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	6,441,624	5,974,497	5,839,387	5,471,914
Available-for-sale Securities [Member] | USDA Securities [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	0	0	0	0
Available-for-sale Securities [Member] | Institutional Credit [Member] | Farmer Mac Guaranteed Securities [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	6,441,624	5,974,497	5,839,387	5,471,914
Trading Securities [Member] | USDA Securities [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 9,487	$ 9,999	$ 11,558	$ 13,515
Measurement Input, Range of Broker Quotes [Member] | Minimum [Member] | Floating rate auction-rate certificates backed by Government guaranteed student loans | Valuation, Market Approach [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment securities, measurement input	0.960	0.950
Measurement Input, Range of Broker Quotes [Member] | Maximum [Member] | Floating rate auction-rate certificates backed by Government guaranteed student loans | Valuation, Market Approach [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment securities, measurement input	0.960	0.950
Measurement Input, Range of Broker Quotes [Member] | Weighted Average [Member] | Floating rate auction-rate certificates backed by Government guaranteed student loans | Valuation, Market Approach [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment securities, measurement input	0.960	0.950
Measurement Input, Discount Rate [Member] | Minimum [Member] | Institutional Credit [Member] | Farmer Mac Guaranteed Securities [Member] | Valuation, Cost Approach [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment securities, measurement input	0.017	0.030
Measurement Input, Discount Rate [Member] | Minimum [Member] | USDA Guarantees [Member] | USDA Securities [Member] | Valuation, Cost Approach [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
USDA securities, measurement input	0.031	0.032
Measurement Input, Discount Rate [Member] | Maximum [Member] | Institutional Credit [Member] | Farmer Mac Guaranteed Securities [Member] | Valuation, Cost Approach [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment securities, measurement input	0.042	0.044
Measurement Input, Discount Rate [Member] | Maximum [Member] | USDA Guarantees [Member] | USDA Securities [Member] | Valuation, Cost Approach [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
USDA securities, measurement input	0.052	0.052
Measurement Input, Discount Rate [Member] | Weighted Average [Member] | Institutional Credit [Member] | Farmer Mac Guaranteed Securities [Member] | Valuation, Cost Approach [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment securities, measurement input	0.031	0.033
Measurement Input, Discount Rate [Member] | Weighted Average [Member] | USDA Guarantees [Member] | USDA Securities [Member] | Valuation, Cost Approach [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
USDA securities, measurement input	0.049	0.049
Measurement Input, Constant Prepayment Rate [Member] | Minimum [Member] | USDA Guarantees [Member] | USDA Securities [Member] | Valuation, Cost Approach [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
USDA securities, measurement input	0.07	0.07
Measurement Input, Constant Prepayment Rate [Member] | Maximum [Member] | USDA Guarantees [Member] | USDA Securities [Member] | Valuation, Cost Approach [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
USDA securities, measurement input	0.19	0.17
Measurement Input, Constant Prepayment Rate [Member] | Weighted Average [Member] | USDA Guarantees [Member] | USDA Securities [Member] | Valuation, Cost Approach [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
USDA securities, measurement input	0.17	0.16